This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Exoribonucleases: Ribonucleases (RNases) play a central role in a number of RNA cellular processes. These processes include mRNA degradation, maturation, and turnover of stable RNAs, which are vital for the proper functioning of all cells. Our laboratory is focusing on structural studies of exoribonucleases, which hydrolyse RNA at their ends. There are eight exoribonucleases in E. coli, and our initial emphasis is on the structures of the three DEDD family members in these enzymes. The long term goals of this research are to understand the structures and mechanisms of action of all the exoribonucleases in a single organism (E. coli);these studies complement a parallel study to completely determine and characterize the physiological role of all the exoribonucleases in E. coli, now underway in the Deutscher laboratory at the University of Miami School of Medicine. Pseudouridine Synthases: In collaboration with the laboratory of James Ofengand, we are carrying out structural studies of bacterial pseudouridine synthases. These ubiquitous enzymes are responsible for isomerization of specific uridines to pseudouridines in structured RNAs such as tRNAs, ribosomal and splicosomal RNAs, and fall into five protein families. We have obtained structures of two of the ten pseudouridine synthases in E. coli and further studies are in progress to better understand how these enzymes recognize their RNA targets and carry out uridine isomerization.